Dickkopf-1 expression during early bovine placentation and its down-regulation in somatic cell nuclear transfer (SCNT) pregnancies.

The precise role of Dickkopf-1 (Dkk-1) during early bovine trophoblast development and subsequent placentation is not fully understood. Using somatic cell nuclear transfer (SCNT) generated pregnancies as a model of poor placentation we have found that mean levels of Dkk-1 mRNA were 1.5 fold lower in SCNT fetal cotyledon tissue at Day 50 of gestation than those resulting from artificial insemination (AI) and 2 fold lower at Days 100 and 150 (P<0.004). Dkk-1 expression in cotyledon tissue was localized by in situ hybridization to fetal binucleate cells (BNCs). Examining conceptuses from blastocyst stage we show that Dkk-1 mRNA was first evident between Days 15-20 of gestation in trophoblast tissue (when BNCs first appear) prior to the initial expression of the BNC specific bovine placental lactogen (bPL) on Day 20. Dkk-1 mRNA levels were higher than bPL in trophoblast tissue throughout the pre-attachment period (Days 24-31), however, this reversed during cotyledon development with only a subset of the bPL immunoreactive BNCs also containing Dkk-1 protein, suggesting a specific role for Dkk-1 during early placentation. One function of Dkk-1 is as an antagonist of the Wnt signaling pathway and, although Wnt5A and Wnt7A mRNAs were expressed in Day 50 bovine cotyledons, their expression levels were similar between AI and SCNT. In addition, the nuclear localization of beta-catenin, which is an indicator of activation of the Wnt pathway, was also similar between AI and SCNT cotyledon tissue. Transcriptional control of Dkk-1 was not due to changes in DNA methylation levels in the promoter region as methylation levels were no different when comparing AI and SCNT tissues. The decreased expression of Dkk-1 in SCNT cotyledons that are prone to abnormal placentation suggests a role in cotyledon formation but the mechanism and regulatory control is yet to be revealed.